Presently, as secondary batteries of high-energy density, lithium-ion secondary batteries are widely employed that are made using a nonaqueous electrolyte solution in which an electrolyte salt is dissolved in a nonaqueous solvent, so as to carry out charge/discharge by causing lithium ions to migrate between the cathode and anode.
In such lithium-ion secondary batteries, a lithium-transition metal oxide having a layered structure such as of lithium nickel oxide (LiNiO2) and lithium cobalt oxide (LiCoO2) is generally used as the cathode. In addition, a carbon material, lithium metal, lithium alloy, and the like capable of storing and releasing lithium are used as the anode.
In addition, as the nonaqueous electrolyte solution, a solution has been used in which an electrolyte salt such as lithium tetrafluoroborate (LiBF4) and lithium hexafluorophosphate (LiPF6) in a nonaqueous solvent such as ethylene carbonate and diethyl carbonate.
On the other hand, in recent years, investigation has begun into sodium-ion secondary batteries in which sodium ion is employed in place of lithium ion. The anode of this sodium-ion secondary battery is formed by a metal including sodium. Therefore, sodium is necessary when manufacturing sodium-ion secondary batteries. Since the resource deposits of sodium are abundant, provided that secondary batteries employing sodium ion in place of lithium ion can be manufactured, it will be possible to manufacture secondary batteries at low cost.
It is necessary to store and release sodium in the anode in order to realize a sodium-ion secondary battery. In this regard, research results have been reported that sodium can be stored and released in the anode in the case of using hard carbon as the anode active material (e.g., refer to Non-patent Documents 1 and 2).
Although examples of storing and releasing sodium in the anode have been reported in the case of using hard carbon as the anode active material, the storage and release can only be repeated for a few cycles. Therefore, technology that improves the cycle performance of batteries has been sought for the development of a superior sodium-ion secondary battery.
Examples of storing and releasing sodium in an anode have also been reported in Non-patent Document 2 in a case of using hard carbon as the anode active material, similarly to Non-patent Document 1. The technology described in Non-patent Document 2 is different from the technology described in Non-patent Document 1 in the aspect of using ethylene carbonate as the nonaqueous solvent and using NaClO4 as the electrolyte salt. The technology described in Non-patent Document 2 is superior compared to the technology described in Non-patent Document 1 in the aspect of the cycle characteristics of the battery. However, in a case of using ethylene carbonate independently as the nonaqueous solvent, the ethylene carbonate is a solid at room temperature, and thus makes a secondary battery that cannot be used at room temperature. Therefore, it has been demanded to make improvements in this secondary battery such that it can be used at room temperature.
In addition, a sodium-ion secondary battery that uses a specific carbon material as the anode active material has been disclosed (Patent Document 1). The sodium-ion secondary battery described in Patent Document 1 is rechargeable at room temperature due to using a mixed solvent of ethylene carbonate and diethyl carbonate as the nonaqueous solvent. Furthermore, the sodium-ion secondary battery described in Patent Document 1 is able to undergo reversible charge/discharge, and can achieve suitable charge/discharge characteristics. However, although a common carbon material including the carbon material described in Patent Document 1 has a layered structure, a problem exists in that the conductivity is poor in a direction perpendicular to the laminated direction. In addition, when using a carbon material having a layered structure, the volumetric change while charging/discharging is great, and thus a problem arises in that the electrode is damaged by this volumetric change, and if propylene carbonate or the like and an organic solvent contact, delamination will occur in the layered structure, and thus a problem also arises in that the battery performance declines.
Although sodium-ion secondary batteries are useful as shown above, there have been problems in the aspect of conventional sodium-ion secondary batteries not being able to be used at room temperature, and the aspect of the battery performance decline caused by the carbon material used as the anode active material. As a result, a sodium-ion secondary battery has been sought that can be used at room temperature and suppresses a decline in the battery performance originating from the anode active material.
[Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2007-35588
[Non-patent Document 1] Journal of the Electrochemical Society, 148(8) A803-A811 (2001)
[Non-patent Document 2] Electrochimica Acta, 47 (2002) 3303-3307